Udp-glucose aglycon-glucosytransferase

ABSTRACT

The present invention provides DNA molecules coding for UDP-glucose:aglycon-glucosyltransferase conjugating cyanohydrins, terpenoids, phenylderivatives or hexanolderivatives to glucose. Transgenic expression of corresponding genes in plants can be used to influence the biosynthesis of the corresponding glucosides.

The present invention provides DNA molecules coding for a UDP-glucose:aglycon-glucosyltransferase conjugating cyanohydrins, terpenoids, phenylderivatives or hexanolderivatives to glucose. Transgenic expression of corresponding genes in plants can be used to influence the biosynthesis of the corresponding glucosides.

The biosynthetic pathway of dhurrin has been studied in otiolated seedlings of Sorghum bicolor, and was found to involve two membrane-bound multi-functional cytochrome P450s. The amino acid precursor L-tyrosine is hydroxylated twice by the enzyme CYP79A1 (P450_(TYR)) forming (Z)-p-hydroxyphenylacetaldoxime (WO 95/16041), which subsequently is converted by the enzyme CYP71 E1 (P450_(OX)) to the cyanohydrine p-hydroxymandelonitrile (WO 98/40470). Transgenic expression of said enzymes is used to modify, reconstitute, or newly establish the biosynthetic pathway of cyanogenic glucosides or to modify glucosinolate production in plants.

In dhurrin biosynthesis, the cyanohydrin p-hydroxymandelonitrile forms an equilibrium with p-hydroxyberzaldehyde and CN at physiological pH and is conjugated to glucose by a UDP-glucose:aglycon-glucosyltransferase. Plants have a large capability to glucosylate a wide range of different chemical structures, but the number of glucosyltransferases present in plants and the range of substrate specificities are largely unknown. Earlier studies indicate that both narrow and broad substrate specificities can be found. Unfortunately, the difficulties encountered in isolating glucosyltransferases to homogeneity without a simultaneous loss of their biological activity confuse the picture. The difficulties encountered partly reflect that many glucosyltransferases have similar molecular mass, are labile and present in minute amounts. Whereas over one hundred different cDNAs encoding putative, secondary plant metabolism glucosyltransferases are described in publicly accessible databases, only a few of the proteins have been verified. There are no reports of the isolation of a cyanohydrin glucosyltransferase from a cyanogenic plant. The present invention demonstrates that expression of both the UDP-glucose:mandelonitrile-glucosyltransferase and the enzymes CYP79A1 and CYP71 E1 in transgenic plants enables these plants to catalyze the conversion of the amino acid tyrosine to the cyanogenic glucoside dhurrin. Thus, the combined expression of proteins catalyzing the reactions involved in the biosynthesis of cyanogenic glucosides in plants actually establishes the complete pathway for cyanogenic glucoside synthesis in these transgenic plants.

Gene refers to a coding sequence and associated regulatory sequences wherein the coding sequence is transcribed into RNA such as mRNA, rRNA, tRNA, snRNA, double stranded RNA, sense RNA or antisense RNA. Examples of regulatory sequences are promoter sequences, 5′ and 3′ untranslated sequences and termination sequences. Further elements such as introns may be present as well.

Expression generally refers to the transcription and translation of an endogenous gene or transgene in plants. However, in connection with genes which do not encode a protein such as antisense constructs, the term expression refers to transcription only.

The following solutions are provided by the present invention:

-   -   A DNA molecule coding for a         UDP-glucose:aglycon-glucosyltransferase conjugating a         cyanohydrin (like mandelonitrile, p-hydroxymandelonitrile,         acetone cyanohydrine or 2-hydroxy-2-methylbutyronitrile); a         terpenoid (like geraniol, nerol or β-citronellol); a         phenylderivative (like p-hydroxybenzoic acid, benzoic acid,         benzylalcohol, p-hydroxy-benzylalcohol,         2-hydroxy-3-methoxybenzylalcohol, vanillin acid or vanillin) or         a hexanolderivative (like 1-hexanol, trans-2-hexen-1-ol,         cis-3-hexen-1-ol, 3-methyl-3-hexen-1-ol or         3-methyl-2-hexen-1-ol) to glucose as well as the encoded protein         itself;     -   Said DNA molecule coding for glucosyltransferase having the         formula R₁-R₂-R₃, wherein         -   R₁, R₂ and R₃ are component sequences consisting of amino             acid residues independently selected from the group of the             amino acid residues Gly, Ala, Val, Leu, lie, Phe, Pro, Ser,             Thr, Cys, Met, Trp, Tyr, Asn, Gin, Asp, Glu, Lys, Arg and             His, and—         -   R₂ consists of 150 or more amino acid residues the sequence             of which is at least 50% identical to an aligned component             sequence of SEQ ID NO: 1 as determined using the computer             program blastp of the BLAST 2.0 set of similarity search             programs, optional parameters set to the default values     -   Said DNA molecule, wherein R₂ encodes 150-425 amino acid         residues such as amino acids 21 to 445, 168 to 448, or 281 to         448 of SEQ ID NO: 1;     -   Said DNA molecule, wherein R₁ and R₃ consist independently of 0         to 500 amino acid residues;     -   Said DNA molecule, wherein R₁ or R₃ encode one or more         additional component sequences having a length of at least 30         amino acids and being at least 65% identical to an aligned         component sequence of SEQ ID NO: 1, such as amino acids 21 to         55, 142 to 174, or 303 to 343 of SEQ ID NO: 1;     -   Said DNA molecule coding for a protein of 300 to 600 amino acid         residues length such as defined in SEQ ID NO: 2 or the protein         defined in SEQ ID NO: 1;     -   A method for the isolation of such cDNA molecules;     -   A method for producing purified recombinant         UDP-glucose:aglycon-glucosyltransferase conjugating a         cyanohydrin, a terpenoid, a phenylderivative or a         hexanolderivative to glucose;     -   A method for obtaining a transgenic plant as well as the         transgenic plant itself comprising stably integrated into its         genome DNA coding for said protein or DNA encoding sense RNA,         anti sense RNA, double stranded RNA or a ribozyme, the         expression of which reduces expression of said protein.

The Arabidopsis thaliana genome is expected to contain approximately 120 genes encoding glucosyltransferases involved in natural product synthesis as deduced from the current state of the Arabidopsis genome sequencing programme. Other plants are also expected to contain a large number of genes encoding glucosyltransferases. In spite of the presence of numerous glucosyltransferases in S. bicolor, none of these except one exert high specificity towards mandelonitrile and p-hydroxymandelonitrile. The presence of several isoforms of this glucosyltransferase is likely considering the evolution and taxonomical background of sorghum and polyploidal forms. The lability of p-hydroxymandelonitrile and the absence of multiple peaks containing p-hydroxymandelonitrile glucosyltransferase activities in S. bicolor during column chromatography demonstrate that a specific glucosyltransferase (sbHMNGT) is involved in the biosynthesis of the cyanogenic glucoside dhurrin.

The biosynthesis of cyanogenic glucosides proceeds according to a general pathway, i.e. involving the same type of intermediates in all plants. Accordingly, the enzymes catalyzing these processes in different plant species are expected to show significant similarity. This has already been clearly demonstrated for the part of the pathway involving conversion of amino acids to oximes. This part has in all plants tested been demonstrated to be catalyzed by one or more cytochrome P450 enzymes belonging to the CYP79 family. These cytochromos P450 show more than 40% sequence identity at the amino acid level. The initial conversion of the amino acids to oximes in glucosinolate synthesis is also catalyzed by a cytochrome P450 enzyme belonging to the CYP79 family. In line with these previous findings, it is expected that in plants synthesizing cyanogenic glucosides conjugation of glucose to cyanohydrins follows a conserved biochemical pathway involving structurally related glucosyltransferases. The aim of the present invention is to provide DNA molecules coding for a UDP-glucose:aglycon-glucosyltransferase conjugating a number of cyanohydrins, a terpenoids, phenylderivatives, and hexanolderivatives (p-hydroxybenzoic acid, benzoic acid, benzylalcohol, p-hydroxy-benzylalcohol and/or geraniol)? to glucose and to define their general structure in cyanogenic plants on the basis of the amino acid sequence of the S. bicolor UDP-glucose:hydroxymandelonitrile-O-glucosyltransferase and its corresponding gene sequence. Thus the present invention provides DNA molecules coding for a UDP-glucose:aglycon-glucosyltransferase and conjugating a cyanohydrin (like mandelonitrile, p-hydroxymandelonitrile, acetone cyanohydrine or 2-hydroxy-2-methylbutyronitrile); a terpenoid (like geraniol, nerol or β-citronellol); a phenylderivative (like p-hydroxybenzoic acid, benzoic acid, benzylalcohol, p-hydroxy-benzylalcohol, 2-hydroxy-3-methoxybenzylalcohol, vanillic acid or vanillin) or a hexanolderivative (like 1-hexanol, trans-2-hexen-1-ol, cis-3-hexen-1-ol, 3-methyl-3-hexen-1-ol or 3-methyl-2-hexen-1-ol) to glucose having the formula R₁-R₂-R₃, wherein

-   -   R₁, R₂ and R₃ are component sequences consisting of amino acid         residues independently selected from the group of the amino acid         residues Gly, Ala, Val, Leu, Ile, Phe, Pro, Ser, Thr, Cys, Met,         Trp, Tyr, Asn, Gln, Asp, Glu, Lys, Arg, His and optionally any         other amino acid residue which can result from posttranslational         modification within a living cell, and     -   R₂ consists of 150, preferably 250 or more amino acid residues         the sequence of which is at least 50%, preferably at least 55%,         or even more prefered at least 70% identical to an aligned         component sequence of SEQ ID NO: 1.

Typical amino acid residues which can result from posttranslational modification within a living cell are Aad, bAad, bAla, Abu, 4Abu, Acp, Ahe, Aib, bAib, Apm, Dbu, Des, Dpm, Dpr, EtGly, EtAsn, Hyl, aHyl, 3Hyp, 4Hyp, Ide, alle, MeGly, Melle, MeLys, MeVal, Nva, Nle and Orn.

Typically R₂ consists of 150 to 425 amino acid residues, a length of 150 to 280 amino acid residues being preferred. Specific embodiments of R 2 are represented by amino acids 21 to 445, 168 to 448 or 281 to 448 of SEQ ID NO: 1.

R1 and R3 independently consist of 0 to 500, preferably 0 to 350 amino acid residues and may comprise one or more additional component sequences having a length of at least 30 amino acids and being at least 65%, but preferably at least 70% identical to an aligned component sequence of SEQ ID NO: 1. Examples of such additional component sequences are represented by amino acids 21 to 55, 142 to 174 or 303 to 343 of SEQ ID NO: 1. The glycosyltransferases encoded by said DNA molecules generally consist of 300 to 600 amino acid residues, the S. bicolor enzyme having a size of 492 amino acid residues as described in SEQ ID NO: 1 and as encoded by SEQ ID NO: 2.

In general there exist two approaches towards sequence alignment. Dynamic programming algorithms as proposed by Needleman and Wunsch and by Sellers align the entire length of two sequences providing a global alingment of the sequences. The Smith-Waterman algorithm on the other hand yields local alignments. A local alignment aligns the pair of regions within the sequences that are most similiar given the choice of scoring matrix and gap penalties. This allows a database search to focus on the most highly conserved regions of the sequences. It also allows similiar domains within sequences to be identified. To speed up alignments using the Smith-Waterman algorithm programs such as BLAST (Basic Local Alignment Search Tool) and FASTA place additional restrictions on the alignments.

Within the context of the present invention overall sequence alignments are conveniently performed using using the program PILEUP available from the Genetic Computer Group, Madison, Wis.

Local alignments are performed conveniently using BLAST, a set of similarity search programs designed to explore all of the available sequence databases regardless of whether the query is protein or DNA. Version BLAST 2.0 (Gapped BLAST) of this search tool has been made publicly available on the internet (currently http://www.ncbi.nim.nih.gov/BLAST/). It uses a heuristic algorithm which seeks local as opposed to global alignments and is therefore able to detect relationships among sequences which share only isolated regions. The scores assigned in a BLAST search have a well-defined statistical interpretation. Particularly useful within the scope of the present invention are the blastp program allowing for the introduction of gaps in the local sequence alignments and the PSI-BLAST program, both programs comparing an amino acid query sequence against a protein sequence database, as well as a blastp variant program allowing local alignment of two sequences only. Said programs are preferably run with optional parameters set to the default values.

Additionally, sequence alignments using BLAST can take into account whether the substitution of one amino acid for another is likely to conserve the physical and chemical properties necessary to maintain the structure and function of a protein or is more likely to disrupt essential structural and functional features. Such sequence similarity is quantified in terms of of a percentage of ‘positive’ amino acids, as compared to the percentage of identical amino acids and can help assigning a protein to the correct protein family in border-line cases.

Investigations into the quantitative and qualitative substrate specificity of sbHMNGT showed a strong preference for the cyanohydrin present in S. bicolor. Thus, in vivo cyanohydrin glucosyltransferases show strong preferences for a limited number of cyanohydrins, terpenoids, phenylderivatives and hexanolderivatives. Nevertheless enzymes catalyzing reactions at the end of biosynthetic pathways often have a broader substrate specificity than those catalyzing preceding reactions resulting in greater flexibility with respect to the evolution of novel secondary metabolite biosynthesis and xenobiotic catabolism. This is illustrated by the finding that whilst the first enzyme of the pathway (CYP79A1) is exclusive for tyrosine, CYP71 E1 and sbHMNGT also accept phenylalanine derived oximes and cyanohydrins, respectively. The presence of a nitrile group is also not necessarily required for substrate recognition by sbHMNGT, as demonstrated by the ability of sbHMNGT to glucosylate benzyl alcohol, benzoic acid, vanillic acid, vanillin and 2-hydroxy-3-methoxybenzylalcohol, geraniol, nerof and β-citronellol. The results demonstrate that sbHMNGT accepts substrates which are structurally similar to the mandelonitrile or p-hydroxy-mandelonitrile. This group of substrate compounds also includes Green Note Flavours such as hexan-1-ol, trans-2-hexene-1-ol and cis-3-hexene-1-ol and other tyrosine or phenylalanine related aroma compounds like phenylacetic acid, phenylethylalcohol, and phenylethylacetate (Krings et al, Appl. Microbiol. Biotechnol. 49: 1-8, 1998). The rates observed for glucosylation of benzyl alcohol, benzoic acid and geraniol are lower than those observed for the cyanohydrins. However, they are still high. To this date there are no reports on the isolation or cloning of a monoterpenoid glucosyltransferase nor of glucosyltransferases for hexanol or hexanol derived compounds, despite the obvious importance of these enzyme classes in defining taste and aroma of processed foods and vegetables.

In the process of glycosylation, unstable compounds (aglycons) are generally rendered less chemically reactive and more water soluble through the enzymatic addition of sugar groups. This typically enables the plant to store increased amounts of these aglycons in the form of glycosides. Many of the secondary metabolites synthesised by plants are glycosylated. For instance over 1500 glycosides of flavonoids alone have been characterised. Glycosylation generally occurs as a late or the last step in the biosynthesis of compounds otherwise unstable in the cellular environment, and can provide a pool of inactive and transportable precursor forms of compounds that can be obtained in an active form by hydrolysis with glucosidase enzymes. Conversion of free aglycons such as terpenoids and Green Note Flavours into corresponding glucosides through the introduction of a glucosyltransferase can be used to preserve aroma, flavour and colour components in fruits, vegetables and other plants. The aglycons can be liberated by the action of specific or unspecific b-glusidases during food preparation or consumption. Further optimization of the catalytic properties towards individual desired aroma, flavours or colour compounds may be achieved through directed evaluation or methods of genetic engineering such as gene shuffling or mutation.

For example in the grapevine the glucosylation of many secondary metabolites has recently become the focus of significant research efforts arising from the discovery that many of the aroma, flavour and colour components of wine are derived from grape compounds which occur in large part as glucosides. Among such target compounds are the terpenes, e.g. geraniol which is found in both a free and a glucosylated form. In view of the present invention the glucoside pool of aroma and flavour precursors can be modulated through manipulation of glucosyltransferase activities and aroma and flavour can be released from stored pools of glucosides via acid or enzyme mediated hydrolysis. Thus, in the grape berry and other fruits, vegetables and plants, the introduction of specific glucosyltransferases such as the cloned sbHMGT or reduction of their expression through anti-sense techniques allows directed modification of secondary metabolite composition. This permits modulatation of important free and bound flavour pools of plants allowing the design of fruits, wines and other plant derived products with defined, organoleptic properties.

The ability of a glucosyltransferase to conjugate an aglycon to glucose can for example be determined in an assay comprising the following steps:

-   a) Incubation of a reaction mixture comprising ¹⁴C-UDP-glucose,     aglycon and UDP-glucose:aglycon-glucosyltranslerase at 30° C.     between 2 minutes and 2 hours -   b) terminating the reaction, and -   c) chemical identification and quantification of the glucoside     produced.

Typically the reaction mixture has a volume of 5 to 2000 μl, but preferably 20 μl and includes 10-200 mM Tris HCl (pH 7.9); 1-5 μM ¹⁴C-UDP-glucose (about 11.0 GBq mmol⁻¹); 0-300 μM UDP-glucose; 0-20 mM aglycone; 25 mM γ-gluconolactone; 0-2 μg/μl BSA and 0-10 ng/μl UDP-glucose:aglycon-glucosyltransferase. β-glucosidase inhibitors other than γ-gluconolactone and protein stabilizers other than BSA may be included as appropriate. One possibility to terminate the reaction is to acidify the reaction mixture for example by adding 1/10 volume of 10% acetic acid.

Chemical identification and quantification of the glucoside formed in the reaction mixture may be achieved using a variety of methodologies including NMR spectroscopy, TLC analysis, HPLC analyses or GLC analysis in proper combinations with mass spectrometric analysis of the glucoside.

Reaction mixtures for analysis by NMR spectroscopy usually have a total volume of 0.5-1 ml, are incubated for 2 hours and include 0-10 mM aglycon, e.g. 2 mM p-hydroxy-mandelonitrile or 6.5 mM geraniol, 3 mM UDP-glucose, 2.5 μg recombinant sbHMNGT, and 0.5 mg BSA. Glucosides are extracted for example with ethyl acetate and lyophillized prior to NMR analysis.

For TLC analysis the reaction mixtures are applied to Silica Gel 60F254 plates (Merck), dried and eluted in a solvent such as ethyl acetate:acetone:dichloromethane: methanol: H₂O (40:30:12:10:8, v/v). Plates are dried for one hour at room temperature and exposed to storage phosphorlmaging plates prior to scanning on a Phosphorlmager. Based on the specific radioactivity of the radiolabelled UDP-glucose, the amount of glucoside formed is quantified.

The radioactivity may also be determined by liquid scintillation counting (LSC analysis). In some cases, where the glucoside formed is derived from a very hydrophobic aglycon, e.g. mandelonitrile, the glucoside can be extracted into an ethyl acetate phase and thereby be separated from unincorporated ¹⁴C-UDP-glucose. 2 ml of scintillation cocktail are added to 250 μl of each ethyl acetate extract and analyzed using a liquid scintillation counter. During column fractionation, those fractions containing sbHMNGT activity can be identified using mandelonitrile as the aglycon substrate and ethyl acetate extraction of the glucoside formed.

Knowledge of SEQ ID NO: 1 and SEQ ID NO: 2 can be used to accelerate the isolation and production of DNA molecules coding for a UDP-glucose:aglycon-glucosyltransferase conjugating cyanohydrins, terpenoids, phenylderivatives or hexanolderivatives to glucose which method comprises

-   (a) preparing a cDNA library from plant tissue expressing     UDP-glucose:aglycon-glucosyltransferase, -   (b) using at least one oligonucleotide designed on the basis of SEQ     ID NO: 2 or SEQ ID NO: 1 to amplify part of the     UDP-glucose:aglycon-glucosyltransferase cDNA from the cDNA library, -   (c) optionally using one or more oligonucleotides designed on the     basis of SEQ ID NO: 2 or SEQ ID NO: 1 to amplify part of the     UDP-glucose:aglycon-glucosyltransferase cDNA from the, cDNA library     in a nested PCR reaction, -   (d) using the DNA obtained in steps (b) or (c) as a probe to screen     the DNA library prepared from plant tissue expressing     UDP-glucose:aglycon-glucosyltransferase, and -   (e) identifying and purifying vector DNA comprising an open reading     frame encoding a protein characterized by an amino acid component     sequence of at least 150 amino acid residues length having 50% or     more sequence identity to an aligned component sequence of SEQ ID     NO: 2, and -   (f) optionally further processing the purified DNA to achieve, for     example, heterologous expression of the protein in a microorganism     like Escherichia coli or Pichia pastoris for subsequent isolation of     the glucosyltransferase, determination of its substrate specificity     and generation of an antibody.

In process steps (b) and (c) the second oligonucleotide used for amplification is preferably an oligonucleotide complementary to a region within in the vector DNA used for preparing the cDNA library. However, a second oligonucleotide designed on the basis of the sequence of SEQ ID NO: 2 or SEQ ID NO: 1 can also be used. A prefered embodiment of this method for the isolation of cDNA is described in Example 4. cDNA clones coding for UDP-glucose:aglycon-glucosyltransferase or fragments of this clone may also be used on DNA chips alone or in combination with the cDNA clones encoding proteins belonging to the CYP79 or CYP71 E1 family of proteins or fragments of these clones. This provides an easy way to monitor the induction or repression of cyanogenic glucoside synthesis in plants as a result of biotic and abiotic factors.

A further embodiment of the present invention are UDP-glucose:aglycon-glucosyl-transferases conjugating a cyanohydrin to glucose such as the S. bicolor enzyme conjugating p-hydroxymandelonitrile to glucose.

Purified recombinant UDP-glucose:aglycon-glucosyltransferases can be obtained by a method comprising dye chromatography and elution with UDP-glucose. An appropriate column material for dye chromatography is Reactive Yellow 3 preferably cross-linked on beaded agarose. Elution of the protein is conveniently achieved using 2 mM UDP-glucose.

The present invention also provides nucleic acid compounds comprising an open reading frame encoding the novel proteins according to the present invention. Said compounds are characterized by the formula R_(A)-R_(B)-R_(C), wherein—

-   -   R_(A), R_(B) and R_(C) constitute component sequences consisting         of nucleotide residues independently selected from the group of         the nucleotide residues G, A, T and C or the group of nucleotide         residues G, A, U and C, —     -   R_(A) and R_(C) consist independently of 0 to 1500, preferably 0         to 1050 nucleotide residues; —     -   R_(B) consists of 450-1260 and preferably 450-840 nucleotide         residues; and     -   the component sequence R_(B) is at least 65% identical to an         aligned component sequence of SEQ ID NO: 2.

Specific examples of the component sequence R B are represented by nucleotides 61 to 1335, 502 to 1344, or 841 to 1344 of SEQ ID NO: 2.

In a preferrred embodiment of the present invention at least one of the component sequences R_(A) or R_(C) comprises one or more additional component sequences which have a length of at least 150 nucleotide residues and are at least 60% identical to an aligned component sequence of SEQ ID NO: 2. Specific examples of such additional component sequences are represented by nucleotides 61 to 165, 427 to 522, or 907 to 1029 of SEQ ID NO: 2.

The pathway for dhurrin synthesis can be introduced into acyanogenic plants by expression of CYP79A1, CYP71 E1 and the sbHMNGT. These three gene products derived from the same plant species, i.e. sorghum, assemble as a macromolecular complex resulting in stronger channeling of the intermediates in the pathway and less free intermediates are released into the plant.

Expressed as transgenes the DNA molecules encoding glycosyltransterases according to the present invention are particularly useful to modify the biosynthesis of cyanogenic glucosides in plants. When the gene encoding a UDP-glucose:cyanohydrin glucosyltransferase is expressed in conjunction with genes encoding cytochrome P450 enzymes belonging to the CYP79 family (catalyzing the conversion of an amino acid to the corresponding N-hydroxyamino acid and the oxime derived from this N-hydroxy amino acid or a cytochrome P450 monooxygenase) and CYP71E family (catalyzing the conversion of an aldoxime to a nitrile and the conversion of said nitrile to the corresponding cyano hydrin), acyanogenic wild-type plants are converted into cyanogenic plants. Proper selection of promoters to provide constitutive, inducible or tissue specific expression of the genes provides means to obtain transgenic cyanogenic plants with desired disease and herbivor responses. Likewise, the content of cyanogenic glucosides in cyanogenic plants may be modified or reduced using anti-sense, double stranded RNA (dsRNA) or ribozyme technology using the same genes. Cyanogenic glucosides belong to the group of phytoanticipins. In cyanogenic plants, blockage or reduction of UDP-glucose:cyanohydrin glucosyltransterase activity is expected to result in production and accumulation of the same products as normally produced by degradation of cyanogenic glucosides in damaged or infected plant cells. Thus using anti-sense or ribozyme technology, plants can be obtained that produce the degradation products of cyanogenic glucosides in the same tissues where cyanogenic glucosides are produced in the wild-type plant resulting in plants with an altered resistance to pathogens and herbivors. Thus, it is a further aspect of the present invention to provide transgenic plants comprising stably integrated into the genome DNA coding for a UDP-glucose:aglycon-glucosyltransferase conjugating cyanohydrins, terpenoids, phenylderivatives or hexanolderivatives to glucose or DNA encoding sense RNA, anti sense RNA, double stranded RNA or a ribozyme, the expression of which reduces expression of a UDP-glucose:aglycon-glucosyltransferase conjugating p-hydroxymandelonitrile to glucose. Such plants can be produced by a method comprising

-   (a) introducing into a plant cell or tissue which can be regenerated     to a complete plant, DNA comprising a gene expressible in that plant     encoding a UDP-glucose:aglycon-glucosyltransferase conjugating     cyanohydrins, terpenoids, phenylderivatives or hexanoldorivatives to     glucose or DNA encoding sense RNA, anti sense RNA or a ribozyme, the     expression of which reduces the expression of a     UDP-glucose:aglycon-glucosyltransferase conjugating a cyanohydrin to     glucose; and -   (b) selecting transgenic plants.

EXAMPLES Example 1 UDP-glucose:p-hydroxymandelonitrile-glucosyltransferase assay

Generally a 20 μl reaction mixture including

-   -   100 mM Tris HCl (pH 7.9),     -   1-5 μM ¹⁴C-UDP-glucose (11.0 GBq mmol⁻¹, Amersham LIFE SCIENCE),     -   0-300 μM UDP-glucose,     -   0-20 mM p-hydroxymandelonitrile (dissolved in water, freshly         prepared),     -   25 mM γ-gluconolactone,     -   0-1 mg BSA and     -   0.5-10 μl of protein preparation,         is incubated at 30° C. between 2 minutes and 2 hours. Thereafter         the reaction is terminated by the addition of 1/10 of the         reaction volume of 10% acetic acid. The same assay conditions         are used to determine the glucosylation of mandelonitrile,         benzoic acid, benzylalcohol, geraniol and a number of other         aglycons.

To determine the substrate specificity of recombinant sbHMNGT incubation lasts for 20 min at 30° C. and the general protocol above is adapted to include

-   -   1.25 mM aglycone (dissolved in ethanol except for flavonoids         which are dissolved in ethylene glycol monoether),     -   1.25 μM ¹⁴C-UDP-glucose,     -   12.5 μM UDP-glucose,     -   100 ng recombinant sbHMNGT, and     -   4 μg BSA.

Quantitative determination of the activity of recombinant sbHMNGT is carried out using 4 minutes incubation at 30° C. Analyses are carried out as for the determination of substrate specificity except that the reaction mixtures are composed as folows:

-   -   1, 5 or 10 mM aglycone,     -   5 μM ¹⁴C-UDP-glucose,     -   0.2 mM UDP-glucose,     -   200 ng recombinant sbHMNGT, and     -   24 μg BSA.

Reaction mixtures for analysis by NMR spectroscopy are incubated for 2 hours in a total volume of 0.5-1 ml including

-   -   2 mM p-hydroxymandelonitrile or 6.5 mM geraniol,     -   3 mM UDP-glucose,     -   2.5 μg recombinant sbHMNGT, and     -   0.5 mg BSA.

Glucosides are extracted with ethyl acetate and lyophillized using speedy-vac prior to NMR analysis.

For TLC analysis the reaction mixture is applied to Silica Gel 60 F254 plates (Merck), dried and eluted in a solvent containing ethyl acetate:acetone:dichloromethane:methanol:H 20 (40:30:12:10:8, v/v). Plates are dried for one hour at room temperature and exposed to storage phosphorlmaging plates (Molecular Dynamics) prior to scanning on a Storm 860 Phosphorimager (Molecular Dynamics).

For analysis by liquid scintillation counting (LSC) reaction mixtures are extracted with 400 μl of ethyl acetate to separate glucosides from unincorporated ¹⁴C-UDP-glucose. 2 ml of Ecoscint A (National Diagnostics, New Jersey, USA) are added to 250 μl of each ethyl acetate extract and analyzed using a Win Spectral 1414 (Wallac) liquid scintillation counter. Mandelonitrile is used as substrate to assay fractions generated by liquid chromatography.

Example 2 Purification of UDP-glucose:p-hydroxymandelonitrile-glucosyltransferase

Except where indicated all steps are carried out at 4° C. Although the endogenous substrate of sbHMNGT is p-hydroxymandelonitrile, mandelonitrile is employed as the substrate for the assay of sbHMNGT activity throughout purification, since it is an equally good substrate. Furthermore, the absence of a hydroxyl group at the para-position of the benzene ring rules out the possibility of p-glucosyloxymandelonitrile synthesis, which would be indistinguishable from dhurrin using the LSC assay.

1 kg of S. bicolor seeds are soaked in water over night at room temperature and subsequently grown for 2 days at 30° C. in darkness as described in (Halkier et al, Plant Physiol. 90: 1552-1559, 1989). Seedling shoots are harvested and extracted in 2 volumes of ice-cold extraction buffer (250 mM sucrose; 100 mM Tris HCl (pH 7.5); 50 mM NaCl; 2 mM EDTA; 5% (w/v) of polyvinylpolypyrrolidone; 200 μM phenylmethylsulfonyl fluoride; 6 mM DTT) using mortar and pestle. The extract is filtered through a nylon mesh prior to centrifugation at 20,000×g for 20 min. The supernatant fraction is subjected to differential ammonium sulphate fractionation (35-70%) with 1 hour precipitations and centrifugations at 20,000×g for 20 min. The pellet is resuspended in buffer A (20 mM Tris HCl (pH 7.5); 5 mM DTT) using a paint brush and desalted using a 100 ml Sephadex G-25 (Pharmacia) or Biogel P-6 (Bio-Rad) column (20 ml/min flow-rate) equilibrated in buffer A. Whilst these purification steps do not result in a measurable increase of the specific activity of sbHMNGT, low molecular weight solutes (including cyanide-precursors) are effectively removed. The first UV-absorbing peak is collected and applied to a 20 ml Q-sepharose (Pharmacia) column (60-80 ml/hr flow-rate) equilibrated in buffer B (buffer A+50 mM NaCl). The column is washed with buffer B until the baseline has stabilised and proteins are eluted with a linear gradient from 50 to 400 mM NaCl in buffer A (800 ml total). 10 ml fractions are collected and 3-5 μl assayed for mandelonitrile glucosyltransferase activity by LSC. All sbHMNGT activity bound to O-sepharose is eluted between 150-200 mM NaCl with a -7-fold purification. Combined active fractions are diluted five-fold in buffer B and concentrated 20-fold using an Amicon YM30 or YM10 membrane prior to storage at −80° C.

The remaining steps of the dye chromatography purification are carried out at room temperature or at 4° C. One quarter of combined concentrated ion-exchange fractions (˜10-15 mg protein in 5 ml) is applied to a column (1 cm×10 cm) containing Reactive Yellow 3 cross-linked on 4% beaded agarose (Lot 63H9502; Sigma) equilibrated in buffer B (10-15 ml/hr). The column is washed with buffer B until the baseline has stabilised. Proteins are eluted with 10 ml of 2 mM UDP-glucose in buffer B. Active fractions containing essentially pure sbHMNGT are pooled and stored at −80° C. with or without addition of 1 mg/ml BSA.

Results: Initial experiments indicated that a 2-day germination period was optimal with regards to total sbHMNGT activity, protein concentration and extract volume. The use of a Waring blender resulted in less than 50% of the activity as compared to extraction with mortar and pestle. sbHMNGT activity was largely unaffected by freezing at −80° C. and the addition of glycerol had no effect. The addition of elevated concentrations of DTT in buffer solutions (5 mM compared to 2 mM) resulted in a ten-fold greater activity after storage at 4° C. for 2 days. This pronounced effect of DTT was primarily found in crude preparations, wheras partially purified ion-exchange preparations were less responsive to the concentration of reducing agents.

Several pseudoaffinity reagents were tested out in mini-column format including Cibachron blue 3G, Reactive Green 19, Reactive Yellow 3 and UDP-glucoronic acid cross-linked with 4% beaded agarose. Trials with elution using NaCl and UDP-glucose at varying salt concentrations identified Reactive Yellow 3 as the superior column material. sbHMNGT activity binds to the Reactive Yellow 3 at 50 mM NaCl and could be eluted after washing with a slight increase in NaCl concentration, without any measurable UV absorbance in the eluate. sbHMNGT activity binds at either salt concentration and can be eluted after washing with a slight increase in NaCl concentration, without any measurable UV-absorbance in the eluate. sbHMNGT activity correlates with a polypeptide migrating around 50-55 kDa by SDS-PAGE, although there are several impurities present (data not shown). Elution with 2 mM UDP-glucose instead of NaCl results in the elution of a similarly migrating polypeptide in apparent homogeneity. When the protocol is repeated it was found that a low column height in relation to total protein was crucial in order to obtain the same degree of purity. Assuming that all of the polypeptide which was visualised by SDS-PAGE was active (and therefore that all inactive protein had been lost) and compensating for cold substrate dilution (UDP-glucose), sbHMNGT represented approximately 0.25% of total protein and was purified 420-fold with a yield of 22%.

Example 3 Peptide Generation and Sequencing

Approximately 5 μg of sbHMNGT is subjected to N-terminal sequencing using a protein sequencer (mode! G1000A, Hewlett-Packard). For peptide digestion, approximately 100 μg of sbHMNGT are precipitated with trichloracetic acid and resuspended in 50 μl of 50 mM Tris.HCl (pH 8.0), 5 mM DTT and 6.4 M Urea. The preparation is incubated at 60° C. for 50 min, cooled to room temperature, and diluted with 3 volumes of 30 mM Tris (pH 7.7) and 1.25 mM EDTA. Endo Lys-C (Promega) is added at a 1:25 ratio (w/w) and the reaction mixture is allowed to incubate for 24 hours at 37° C. Peptides are purified by reverse-phase HPLC using a Vydac 208TP52 C8 column (250 mm×21 mm) and Beckman System Gold HPLC equipment. Peptides are applied at a 0.2 m/min flow-rate in buffer C (0.1% trifluoroacetic acid) and eluted with a linear gradient from 0 to 80% acetonitrile in buffer C. Fractions are collected manually and sequenced as described above.

Example 4 Cloning

PCR amplification: 1st round PCR amplification reactions are carried out using 2 units of Taq DNA polymerase (Pharmacia), 4 μl of 10×Taq DNA polymerase buffer, 5% (w/w) dimethyl sulfoxide, 1 μl dNTPs (10 mM), 80 pmoles each of primers C2EF (5′-TTYGTIWS-ICAYTGYGGITGGAA-3′, SEQ ID NO: 3) and T7 (5′-AATACGACTCACTATAG-3′, SEQ ID NO: 4) and about 10 ng of plasmid DNA template in a total volume of 40 μl. The plasmid DNA template is prepared from a unidirectional pcDNAII (Invitrogen) plasmid library made from 1-2 cm high etiolated S. bicolor seedlings (Bak et al, Plant Mol. Biol. 36: 393-405, 1998). Thermal cycling parameters are 95° C., 5 min, 3×(95° C. for 5 sec, 42° C. for 30 sec, 72° C. for 30 sec), 32×(95° C. for 5 sec, 50° C. for 30 sec, 72° C. for 30 sec) and a final 72° C. for 5 min.

2nd round PCR amplifications are carried out as above, except for using primers C2DF (5′-GARGCIACIGCIGCIGGICARCC-3′, SEQ ID NO: 5) and T7, and 1 μl of 1st round reaction as DNA template. Thermal cycling parameters are 95° C., 5 min, 32×(95° C. for 5 sec, 55° C. for 30 sec, 72° C. for 30 sec) and a final 72° C. for 5 min. The PCR reaction mixtures are subjected to gel electrophoresis using a 1.5% agarose gel and an approximately 600 bp band is excised and cleaned using a Qiaex II gel extraction kit (Qiagen). The cleaned PCR product is then ligated into the pGEM-T vector and used to transform the E coli JM109 strain according to the manufacturers instructions (Promega). Nucleic acid sequencing reveals the presence of two previously obtained peptide sequences in the translation product of PCR clone 15#44.

Cloning and Library Screening: The PCR clone 15#44 is used as a template for generating a 306 bp digoxigenin-11-dUTP-labelled probe by PCR using primers 441 F (5′-GAGGCGA-CGGCGGCGGGGCAG-3′, SEQ ID NO: 6) and 442R (5′-CATGTCACTGCTTGCCCCCGACCA-3′, SEQ ID NO: 7) according to the manufacturer's instructions (Boehringer Mannheim). The labelled probe is cleaned using the Qiaex II gel extraction kit after gel electrophoresis with a 1.5% agarose gel and employed to screen approximately 50,000 colonies of the abovementioned plasmid library. Hybridizations are carried out over night at 65° C. in 5×SSC, 0.1% (w/v) N-lauroylsarcosine, 0.02% (w/v) SDS and 1% blocking reagent (Boehringer Mannheim). Membranes are then washed in 0.5×SSC at 60° C., 3×15 min.

Seven hybridizing clones are isolated and one full-length clone, sbHMNGT1, is chosen for further characterization.

Example 5 Identity and Similarity Between sbHMNGT and Translation Products of Known or Putative glucosyltransferase-Encoding cDNAs

Table 1 summarizes the overall identity respectively similarity between sbHMNGT and known or putatice glycosyltransferase amino acid sequences as well as the identities respectively similarities in the corresponding N-terminal regions, i.e. the region defined as the sequence N-terminal of the consensus sequence xCLxWL with the split-point being at amino acid residue 291/292 of sbHMNGT.

Table 2 summarizes the similarity respectively identity between the amino acid sequence of sbHMNGT region α, defined as residues 188-229 in HMNGT, and corresponding sequences in known or putative glycosyltransferase amino acid sequences.

The calculations of similarity and identity are based on a pairwise comparisons of cDNA translation products using the GAP program (Genetic Computer Group, Madison, Wis.), wherein A/G, Y/F, S/T, V/I/L, R/K/H, and D/E/N/Q are considered to constitute similar residues. Abbreviated sequence names are stSGT (Solanum tuberosum solanidine-glucosyltransforase: GenBank™ accession number U82367); bnTHGT (Brassica napus thiohydroximate-S-glucosyltransferase: SEQ ID NO: 28 of EP-771 878-A1), zmUFGT (Maize flavonoid-glucosyltransferase: GenBank™ accession number X13502), vvUFGT (Vitis vinifera anthocyanidin-glucosyltransferase: GenBank™accession number AF000371), psGT (Pisum sativum UDP-glucuronosyltransferase: GenBank™ accession number AF034743), meGT (Cassaya UTP-glucose glucosyltransferase: GenBank™ accession number X77464), and zmlAAGT (Maize Indole-3-acetate beta-glucosyltransferase: GenBank™ accession number L34847). TABLE 1 sbHMNGT Overall % N-terminal % Identity Similarity Identity Similarity zmUFGT 36.7 41.5 32.6 37.1 vvUFGT 30.0 38.7 23.8 33.3 psGT 41.6 51.5 32.9 46.3 meGT 31.3 41.6 25.3 36.8 zmIAAGT 34.9 41.3 27.8 35.0 snSGT 28.9 38.0 23.6 31.0 bnTHGT 30.7 38.0 24.7 33.3

TABLE 2 α region identities (italic) and similarities (bold face) sbHMNGT psGT zmUFGT vvUFGT mhGT sbHMNGT 45.2% 26.2% 19.1% 20% psGT 69.1% — — — zmUFGT 35.7% — 47.6% 37.5%   vvUFGT 35.7% — 59.5% — mhGT 37.5% — 55.0% —

Example 6 Heterologous Expression

Primers EXF1 (5′-AATAAAAAGCATATGGGAAGCAACGCGCCGCCTCCG-3′, SEQ ID NO: 8) and EXR1 (5-TTGGATCCTCACTGCTTGCCCCCGACCA-3′, SEQ ID NO: 9) are used to amplify a 1500 bp full-length sbHMNGT insert by PCR, using the sbHMNGT1 plasmid as template. The primers contain 5′ recognition sites for restriction endonucleases NdeI (EXF1) and BamHI (EXR1). PCR reaction conditions are essentially as described in example 4, except for the thermal cycling parameters which are 95° C., 3 min, 30×(95° C. for 5 sec, 53° C. for 30 sec, 72° C. for 90 sec) and a final 72° C. for 5 min. The PCR product is gel purified, digested with NdeI and BamHI, gel purified once again and ligated into the plasmid expression vector pSP19g10L (Barnes, Methods in Enzymology 272: 3-14, 1996) which has also been digested with the restriction enzymes NdeI and BamHI and gel purified. The ligation reaction mixture is then used to transform E. coli JM109 cells according to the manufacturers instructions (Promega). Alter selection of successfully cloned cells, expression is initiated as described in (Ford et al, J. Biol. Chem. 273: 9224-9233, 1998). Briefly, 600 μl of a 37° C. over night culture are added to 300 ml luria broth (LB) containing 100 μg/ml ampicillin. The culture is allowed to grow at 28° C. under continuous shaking at 150 rpm for 5 hours and IPTG is then added to a final concentration of 0.4 mM. After induction the culture is allowed to continue growing over night and harvested by centrifugation at 2500×g for 10 min. The pellet is resuspended in 9 ml of 200 mM Tris pH 7.9, 1 mM EDTA, 5 mM DTT and 0.1 mg/ml lysozyme. An equal volume of ice-cold water is added and the mixture allowed to incubate for 10 min at RT, followed by 20 min incubation on ice. After the addition of 18 μmoles of phenylmethylsulfonyl fluoride and 100 units of DNasel/ml (Sigma), the suspension is subjected to three freeze and thaw cycles at −20° C. Phenylmethylsulfonyl fluoride is adjusted to 1.5 mM final concentration and the preparation centrifuged at 15,000×g for 15 min. Negative controls, containing no insert in the plasmid vector, are prepared as above. For purification of the recombinant protein two 300 ml cultures are lysed as above and further purified as for the native protein. Briefly, crude cell lysate is subjected to Q-sepahrose chromatography, desalting and Reactive Yellow 3 chromatography as described in example 2. The yield of recombinant protein is approximately 1 mg/100 ml LB culture.

Example 7 Substrate Specificity of Recombinant sbHMNGT Compared to Desalted Ccrude Etiolated Sorghum Seedling Extract

Glucosyltransterase activity was determined by TLC using ¹⁴C-UDP-glucose. Filled boxes in Table 1 below (▪) indicate that a radiolabelled product was visualised after incubation with the respective aglycone substrate. Empty boxes (□) indicate that no radiolablled products could be detected under the experimental conditions employed. Figures in brackets indicate the relative V_(max) for each aglycon with calculated standard deviations. The V_(max) value for p-hydroxymandelonitrile was 1500 mol of product/mol of sbHMNGT/sec. TABLE 3 ACTIVITY Crude SUBSTRATES Sorghum Recombinant cyanohydrins extract sbHMNGT 1) mandelonitrile ▪ ▪ (77.8 ± 8.6%) 2) p-hydroxymandelonitrile ▪ ▪ (100 ± 7.2%) 3) acetone cyanohydrin □ □ benzyl derivatives 4) hydroquinone ▪ □ 5) benzyl alcohol ▪ ▪ (13.1 ± 2.1%) 6) p-hydroxybenzyl alcohol ▪ ▪ 7) benzoic acid ▪ ▪ (4.2 ± 0.8%) 8) p-hydroxybenzoic acid ▪ □ 9) p-hydroxybenzaldehyde ▪ □ 10) gentisic acid □ □ 11) caffeic acid ▪ □ 12) 2-hydroxy cinnamic acid ▪ □ 13) resveratrol (stilbene) ▪ □ 14) salicylic acid ▪ □ 15) p-hydroxymandelic acid ▪ □ 16) vanillic acid ▪ ▪ 17) vanillin ▪ ▪ 18) 2-hydroxy-3-methoxybenzlalcohol ▪ ▪ flavonoids 19) quercetin (flavonol) ▪ □ 20) cyanidin (anthocyanidin) ▪ □ 21) biochanin A (isoflavone) ▪ □ 22) naringenin (flavanone) ▪ □ 23) apigenin (flavone) ▪ □ hexanol derivatives 24) 1-hexanol ▪ ▪ 25) trans-2-hexen-1-ol ▪ ▪ 26) cis-3-hexen-1-ol ▪ ▪ 27) 3-methyl-3-hexen-1-ol ▪ ▪ 28) 3-methyl-2-hexen-1-ol ▪ ▪ others 29) indole acetic acid (plant hormone) ▪ □ 30) geraniol (monoterpenoid) ▪ ▪ (11.0 ± 0.5%) 31) tomatidine (alkaloid) ▪ □ 32) nerol ▪ ▪ 33) p-citronellol ▪ ▪ 

1. An isolated DNA molecule comprising a nucleotide sequence coding for a UDP-glucose:aglycon-glucosyltransferase that conjugates an aglycon selected from the group consisting of a cyanohydrin, a terpenoid, a phenylderivitive and a hexanolderivitive to glucose, wherein the compliment of said nucleotide sequence hybridizes to SEQ ID NO:2 under the following hybridization conditions: hybridization in 5×SSC, 0.1% N-lauroylsarcosine, 0.02% SDS, 1% Blocking Reagent at 65° C., and washing three times for 15 minutes in 0.5×SSC at 60° C.
 2. The isolated DNA molecule according to claim 1, wherein said aglycon is selected from the group consisting of mandelonitrile, p-hydroxymandelonitrile, gerinol, nerol, β-citronellol, benzoic acid, benzylalcohol, p-hydroxy-benzylalcohol, 2-hydroxy-3-methoxybenzylalchol, vanillic acid, vanillin, 1-hexenol, trans-2-hexen-1-ol, cis-3-hexen-1-ol, 3-methyl-3-hexen-1-ol and 3-methyl-2-hexen-1-ol.
 3. The isolated DNA molecule according to claim 1, wherein the nucleotide sequence codes for a UDP-glucose:aglycon-glucosyltransferase comprising the formula R1-R₂-R₃, wherein R₁, R₂ and R₃ are component sequences consisting of amino acid residues independently selected from the group consisting of the amino acid residues Gly, Ala, Val, Leu, Ile, Phe, Pro, Ser, Thr, Cys, Met, Trp, Tyr, Asn, Gln, Asp, Glu, Lys, Arg and His; and R₂ comprises 150 or more amino acid residues the sequence of which is at least 50% identical to an aligned component sequence of SEQ ID NO:
 1. 4. The isolated DNA molecule according to claim 1, wherein the amino acid sequence of R₂ is represented by amino acids 21 to 445, 168 to 448, or 281 to 448 of SEQ ID NO:
 1. 5. The isolated DNA molecule according to claim 1, wherein R₁ or R₃ comprise one or more additional component sequences having a length of at least 30 amino acids and being at least 65% identical to an aligned component sequence of SEQ ID NO:
 1. 6. The isolated DNA molecule according to claim 1, wherein said nucleotide sequence codes for a UDP-glucose:aglycon-glucosyltransferase of 300 to 600 amino acid residues in length.
 7. The isolated DNA molecule according to claim 1, wherein said nucleotide sequence codes for a UDP-glucose:aglycon-glucosyltransferase comprising the amino acid sequence set forth in SEQ ID NO:
 1. 8. The isolated DNA molecule according to claim 1, comprising the nucleotide sequence set forth in SEQ ID NO:
 2. 9. An isolated UDP-glucose:aglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexanolderivative to glucose as coded for by the isolated DNA molecule according to claim
 1. 10. A method for the isolation of a cDNA molecule comprising a nucleotide sequence that codes for a UDP-glucose:aglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexanolderivative to glucose; comprising (a) preparing a cDNA library from plant tissue expressing said UDP-glucose:aglycon-glucosyltransferase, (b) using at least one oligonucleotide designed on the basis of SEQ ID NO: 1 to amplify part of the UDP-glucose:aglycon-glucosyltransferase cDNA from the cDNA library, (c) optionally using a further oligonucleotide designed on the basis of SEQ ID NO: 1 to amplify part of the UDP-glucose:aglycon-glucosyltransferase cDNA from the cDNA library in a nested PCR reaction, (d) using the DNA obtained in steps (b) or (c) as a probe to screen a cDNA library prepared from plant tissue expressing UDP-glucose:aglycon-glucosyltransferase, and (e) identifying and purifying vector DNA comprising an open reading frame encoding a protein characterized by an amino acid component sequence of at least 150 amino acid residues in length having 50% or more sequence identity to an aligned component sequence of SEQ ID NO: 2 or a sequence encoding part of SEQ ID NO: 1 (f) optionally further processing the purified DNA.
 11. A method for producing purified recombinant UDP-glucose:aglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexanolderivative to glucose; comprising (a) obtaining an extract comprising said recombinant UDP-glucose:aglycon-glucosyltransferase; (b) eluting said extract with a linear salt gradient using Q-Sepharose chromatography, resulting in fractions enriched for said UDP-glucose:aglycon-glucosyltransferase by about 7-fold; and (c) eluting said fractions with UDP-glucose using dye chromatography, resulting in purified recombinant UDP-glucose:aglycon-glucosyltransferase.
 12. A transgenic plant comprising stably integrated into its genome DNA comprising a nucleotide sequence coding for a UDP-glucose:aglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexenolderivative to glucose or DNA comprising a nucleotide sequence that encodes sense RNA, antisense RNA, double stranded RNA or a ribozyme, the expression of which reduces expression of a UDP-glucose:alglycon-glucosyltransferase that conjugates p-hydroxymandelonitrile to glucose.
 13. The transgenic plant of claim 12 additionally comprising stably integrated into its genome, DNA comprising a nucleotide sequence coding for a cytochrome P-450 monooxy-genase that catalyzes the conversion of an amino acid to the corresponding N-hydroxyamino acid and the conversion of said N-hydroxyamino acid to the corresponding oxime or a cytochrome P450 monooxygenase that catalyzes the conversion of an aldoxime to a corresponding nitrile and the conversion of said nitrile to the corresponding cyanohydrin.
 14. A method for producing a transgenic plant according to claim 12 comprising (a) introducing into a plant cell or plant tissue that can be regenerated into a complete plant, DNA comprising a gene expressible in said plant encoding a UDP-glucose:aglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexenolderivative to glucose; and (b) selecting transgenic plants.
 15. A method for producing a transgenic plant according to claim 12 comprising (a) introducing into a plant cell or plant tissue that can be regenerated into a complete plant, DNA comprising a gene expressible in said plant encoding sense RNA, antisense RNA, double stranded RNA or a ribozyme, the expression of which reduces expression of a UDP-glucose:alglycon-glucosyltransferase that conjugates a cyanohydrin, a terpenoid, a phenylderivative or a hexenolderivative to glucose; and (b) selecting transgenic plants.
 16. The isolated DNA molecule according to claim 1, wherein said nucleotide sequence is isolated from a plant that produces a cyanogenic glucoside.
 17. The isolated DNA molecule according to claim 16, wherein the cyanogenic glucoside in dhurrin.
 18. The isolated DNA molecule according to claim 16, wherein said plant is selected from the group consisting of the genera Sorghum, Triticum, Secale, and Macadamia.
 19. The isolated DNA molecule according to claim 16, wherein said plant is selected from the group consisting of the species Sorghum bicolor, Triticum aestivum, Secale cereale, and Macadamia ternifolia.
 20. The isolated DNA molecule according to claim 16, wherein said plant is Sorghum bicolor.
 21. A recombinant nucleic acid construct comprising a promoter operatively linked to the DNA molecule according to claim
 1. 22. A transgenic host cell comprising the recombinant nucleic acid construct of claim
 21. 23. The transgenic host cell according to claim 22, which is a bacterial cell or a plant cell.
 24. A transgenic plant comprising the transgenic plant cell of claim
 23. 25. The transgenic plant according to claim 24, wherein said plant is selected from the group consisting of maize, rice, barley, sorghum, cotton, soybeans, sunflower, oil seed rape, sugar beet, broccoli, cauliflower, cabbage, cucumber, sweet corn, tomato, and grape.
 26. The transgenic plant according to claim 24, wherein said plant is a maize plant.
 27. Transgenic seed from the transgenic plant according to claim
 24. 28. An isolated UDP-glucose:aglycon-glucosyltransferase that conjugates an aglycon selected from the group consisting of a cyanohydrin, a terpenoid, a phenylderivative and a hexanolderivative to glucose.
 29. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 28, wherein said aglycon is selected from the group consisting of mandelonitrile, p-hydroxymandelonitrile, gerinol, nerol, β-citronellol, benzoic acid, benzylalcohol, p-hydroxy-benzylalcohol, 2-hydroxy-3-methoxybenzylalchol, vanillic acid, vanillin, 1-hexenol, trans-2-hexen-1-ol, cis-3-hexen-1-ol, 3-methyl-3-hexen-1-ol and 3-methyl-2-hexen-1-ol.
 30. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 28, wherein said UDP-glucose:aglycon-glucosyltransferase is obtained from a plant that produces a cyanogenic glucoside.
 31. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 30, wherein the cyanogenic glucoside in dhurrin.
 32. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 30, wherein said plant is selected from the group consisting of the genera Sorghum, Triticum, Secale, and Macadamia.
 33. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 30, wherein said plant is selected from the group consisting of the species Sorghum bicolor, Triticum aestivum, Secale cereale, and Macadamia ternifolia.
 34. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 28, wherein said UDP-glucose:aglycon-glucosyltransferase comprises an amino acid sequence that has at least 70% sequence identity with SEQ ID NO:
 1. 35. An isolated UDP-glucose:aglycon-glucosyltransferase according to claim 28, wherein said UDP-glucose:aglycon-glucosyltransferase comprises the amino acid sequence set forth in SEQ ID NO:
 1. 36. A method of producing an isolated UDP-glucose:aglycon-glucosyltransferase according to claim 28, comprising: (a) engineering a gene encoding said UDP-glucose:aglycon-glucosyltransferase to be expressible in a host organism; (b) transforming said host organism with the engineered gene; and (c) isolating the UDP-glucose:aglycon-glucosyltransferase from the host organism or a culture supernatant.
 37. The method according to claim 36, wherein the host organism is a beacterial cell or a plant.
 38. The method according to claim 37, wherein said plant is selected from the group consisting of maize, rice, barley, sorghum, cotton, soybeans, sunflower, oil seed rape, sugar beet, broccoli, cauliflower, cabbage, cucumber, sweet corn, tomato, and grape.
 39. The method according to claim 37, wherein said plant is a maize plant. 